This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting coatings on metals, or other materials coated with metals, without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-inhibiting conversion coatings based on trivalent and tetravalent cobalt and methods of making and using the same.
Metals like aluminum, zinc, magnesium, titanium, cadmium, silver, copper, tin, lead, cobalt, zirconium, beryllium, or indium, their alloys, or items coated with these metals, tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials or ease of oxide formation. Non-alloyed specimens of these metals typically form a natural oxide film that will protect them somewhat and reduce their overall rate of corrosion. However, alloys of these metals are particularly sensitive to corrosive attack. These materials also have a significant problem with paint adhesion. The as-formed metal surfaces are typically very smooth, and they tend to form weakly bound surface oxides. The native oxides do not normally provide a robust base on which subsequent paints can anchor themselves. These metal alloys have many uses ranging from architectural adornments to protective coatings on ferrous alloys to structural aerospace components.
The 2000 and 7000 series of aluminum alloys are used throughout military and civilian aircraft because of their high strength to weight ratio. However, these aluminum alloys are very sensitive to corrosive attack because their natural oxide layer offers only a limited degree of protection. Materials with greater redox potentials, such as steels or carbon fibers, in proximity to aluminum alloys will promote corrosive attack in water by the formation of a galvanic corrosion couple with the less-noble light metal alloy.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion.
One method to enhance the corrosion resistance of these alloys is through the use of a conversion coating. A conversion coating is a self-healing, corrosion-inhibiting layer formed during intentional exposure to a chemically reactive solution. The conversion coating process forms an adherent surface containing an integral corrosion inhibitor with “throwing power” that can provide protection to coating breaches. The metal is exposed to a compound that chemically alters the surface and forms a coating that provides a high degree of corrosion resistance. A chemical conversion coating applied to the surface of a less-noble alloy can reduce the extent and severity of aqueous corrosion, provide long-term property stability, and extend the useful life of the object of manufacture.
Conversion coatings incorporate a portion of the base metal and form a mechanical, chemical, and electrostatic barrier to corrosive attack. A critical feature of effective conversion coatings is their ability to provide corrosion protection to the base metal in the presence of a coating breach.
Anodization of a metal surface followed by “sealing” or “rinsing” of the anodized metal does not constitute the formation of a conversion coating in our usage. Anodization, the formation of a porous oxide film on the metal, is achieved by the application of an electrical potential to the metal. This oxide film must then be “sealed”, “washed”, or “rinsed” in order to impart complete corrosion protection. Typically, the corrosion protection afforded by an anodized piece is due to the barrier oxide film. Conversion coatings, however, grow an oxide coating on the metal without an externally applied electrical potential. The protective film is produced by a chemical redox reaction between the metal surface and the conversion coating solution. The film is composed both of an oxide and integral corrosion inhibitor species formed during exposure to the conversion coating solution. A true conversion coating therefore affords corrosion protection from an oxide barrier film that has co-deposited oxidative corrosion inhibitor species.
A conversion-coated surface may be left bare or afforded further protection by the application of additional films or coatings. Conversion coatings need to adhere to the substrate and should result in a surface that will promote the formation of a strong bond with subsequently applied coatings. Bonding with subsequently applied coatings is a function of the morphology and chemical composition of the conversion coating. Adhesion promoting surface treatments may exhibit corrosion inhibiting characteristics. Depending on the intended application, a conversion coating, as described herein, may be considered to be an “adhesion promoter” and vice versa.
Conversion coatings are usually formed by the application of a conversion coating solution to a metal surface. The solution can be applied by immersion, spray, fogging, wiping, or other means.
Hexavalent chromium has traditionally been used in the formation of protective conversion coatings for aluminum, zinc, magnesium, titanium, cadmium, silver, copper, tin, lead, cobalt, zirconium, beryllium, indium, and their alloys. Compounds such as Alodine 1200™ (Henkel Co.) and Alumagold™ (Turco Co.) contain hexavalent chromium as their main corrosion-inhibiting compound.
Two generic types of hexavalent chromium coatings have been widely used. The newer “gold” coatings are named for the faint gold tint that the coatings have when they form on the surface of aluminum alloys. The compositions and application procedures of these “gold” hexavalent chromium conversion coating formulations are described in United States military process specifications, as well as other federal guidelines. Therefore, guidelines for the application of these solutions to aluminum (MIL C-5541; MIL C-81706; MIL STD-171; ASTM B-449), zinc (ASTM B-633; ASTM B-201; MIL C-17711; QQ Z-325a), magnesium (MIL M-3171), cadmium (ASTM A-165; ASTM B-201; QQ P-416b), silver (ASTM B-700; QQ S-365a), copper (ASTM B-281), and tin (ASTM A-599; QQ-T-425a) are available. The common components to these “gold” conversion coating baths are hexavalent chromium, complex fluorides, and ferricyanide. Older “green” conversion coatings containing hexavalent chromium have also been described, and the color formed on aluminum alloys through the application of these conversion coatings is a light green color. The “green” formulations all contain hexavalent chromium, a fluoride, and an acidic phosphate component. The major compositional difference between the two is that the current “gold” formulation contains ferricyanide and the older “green” formulations contain phosphate.
Corrosion-resistant compositions have also been described which contain hexavalent chromium, fluoride, and molybdic acid or molybdates, rather than ferricyanide or phosphate. Tungstates and vanadates have also been used in combination with hexavalent chromium and fluoride. Hexavalent chromium formulations which do not contain a fluoride source, and which contain borate ions instead of ferricyanide or phosphate or molybdate have also been described. Hexavalent chromium has also been used in combination with stannates, oxalates, and tellurates. Finally, corrosion protection of aluminum, magnesium, or zinc alloys has been achieved through the use of hexavalent chromium, fluoride, and rare earth compounds.
The variation in the type and amount of additional components such as ferricyanide, phosphate, molybdate, and borate, etc., in conversion coat formulations based on hexavalent chromium is significant in light of the chemistry developed and presented in the present invention. It is important to note that hexavalent chromium conversion coatings which have nearly identical formulations, except for one or more of the non-chromium components, result in obvious differences on the applied metal surface for a given alloy (such as “gold” and “green” coatings). It is also important to note that differences in the composition of aluminum alloys will influence the chemistry of the conversion coating formed when only one hexavalent chromium conversion coat composition is used.
Significant efforts have been made to replace chromium with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory concerns. Cobalt is one non-toxic, non-regulated metal which has been considered as a chromium replacement. Cobalt (like chromium) exhibits more than one oxidation state (Co+2 and Co+3). In addition, the oxidation-reduction potential of the Co+3—Co+2 couple is comparable to the Cr+6—Cr+3 couple. For example, in acid solution:
Co+3 + e => Co+2 +1.92 VCr+6 + 3e => Cr+3 +1.36 VA number of processes have been reported in the literature which make use of cobalt in conversation coating bath solutions, as well as general corrosion protection or coloring of the alloys. However, the coatings formed by these processes provide only limited protection and do not approach the benefit derived from the use of hexavalent chromium.
The use of film-forming substances, such as polymers, silicates, sol-gel, etc., which have no inherent oxidizing character, in conversion coating solutions has been described in the literature. The film formers may enhance short-term corrosion resistance by functioning as a barrier layer. However, these films interfere with substrate oxidation during the conversion coating process and produce thin, incompletely anodized surfaces, resulting in poor mechanical adhesion to the solution-deposited polymer film and to later applied coatings. Restricting the formation of the oxide layer that acts as a reservoir for the active corrosion inhibitor yields a barrier film that is inhibitor starved. Barrier layers lacking an active corrosion inhibitor have been demonstrated to be capable of inhibiting corrosion only as long as the barrier is not breached, as by a scratch or other flaw. Film formers can actually enhance corrosion on a surface after failure due to the well known effects of crevice corrosion. The addition of polymer during conversion coating also produces a smooth coating which can reduce subsequent paint adhesion, resulting in reduced long-term corrosion protection.
The following references describe conversion coating processes based on cobalt: PCT International Application Nos. WO 96/29,448, WO 98/51,841, WO 96/21,753, WO 93/05,198, and S. African Patent No. ZA 93/01,234 to Dolan; PCT International Application Nos. WO 96/05,335, WO 94/00,619, and European Patent Application Nos. EP 523,288, EP 458,020, EP 488,430, and U.S. Pat. Nos. 5,873,953, 5,411,606, 5,378,293, 5,298,092, and 5,551,994 to Schriever. These specifications use additives that they term “bath stabilizers.” These chemical species are claimed to form more stable coordination bonds with cobalt(III) cations than with cobalt(II) cations in the aqueous conversion coating solution. Specifically, carboxylates, hydroxyalkyl amines (aminoalcohols, such as triethanolamine), or nitrito complexes are described in these specifications as being added to the bath to retain trivalent cobalt in solution and to stabilize concentrations during the coating process. These bath stabilizers only treat and extend the service life of the cobalt(III) in the conversion coating solution itself.
“Bath stabilizers” used in the manner of these specifications reduce the formation and precipitation of Co+3-containing solids during coating deposition. Bath stabilizers described in these patents behave similar to masking agents for chemical gravimetric analysis to keep unwanted compounds from precipitating. They actually serve to starve the deposited coating of Co+3 by shifting the equilibrium away from the formation of a corrosion-inhibiting coating containing trivalent cobalt on a metal surface to the trivalent cobalt remaining dissolved in the coating bath. The compounds formed from the solutions described in these specifications have lower structural stability in the coating, as well as higher aqueous solubility, than if no bath stabilizer were used at all. The art described in the specification and examples herein shows that the solubilities of the formed compounds are still too high to afford long-term corrosion protection.
A post-treatment rinse with a vanadate or tungstate solution is used in PCT International Application Nos. WO 96/29,448 and WO 98/51,841 to Dolan, as well as PCT International Application No. WO 96/05,335 and U.S. Pat. No. 5,551,994 to Schriever. This rinse seals the coating deposited from the solution, as described in these specifications. Co+3-vanadate/tungstate complexes form during these sealing treatments. These complexes are slightly soluble and serve to enhance the corrosion resistance of the deposited coating. However, the sealing step used in this art is not an efficient method to treat the coating thickness or to incorporate sparingly soluble Co+3 compounds into the coating effectively. The effectiveness of the vanadate/tungstate sealing step is also reduced because the bath stabilizers carried over from the first solution increase the solubility of Co+3-vanadate/tungstate complexes. Furthermore, the toxicity of the conversion coatings is not reduced if pentavalent vanadium is used in chrome-free compositions because the threshold limit value (TLV) of both CrO3 and V2O5 is 0.05 mg/m3, and the permissible exposure limit (PEL) of both is 0.5 mg/m3.
These Schriever and Dolan coating processes also require the use of elevated temperatures, especially for the sealing process (40 to 75° C. being a typical range). Conversion coating processes that take place at elevated temperatures (above room temperature) can result in higher coating costs and increase the difficulty of the coating application.
Accordingly, a need exists for improved corrosion-protection conversion coatings composed of currently unregulated and/or non-toxic materials which have an effectiveness, ease of application, and performance comparable to coatings formed with hexavalent chromium, and for methods of making and using the same.